Aromatic polybenzimidazoles are a unique class of intractable, i.e., not melt processable, organic polymers which are high temperature resistant, as well as chemical and oxidative resistant. Because these polymers are not melt processable, it is difficult to form them into film, fibers and molded articles. To enhance the melt processability, reduce the cost of products made therefrom, and moisture takeup of the finished product, polybenzimidazole is often blended with a tractable polymer. One useful tractable polymers useful for blending with polybenzimidazole is aromatic polysulfones. However, desirable mechanical and thermal properties are significantly reduced by the addition of these melt processable polymers; the degree of reduction is usually proportional to the weight percent of the melt processable polymer addition. Blends of polybenzimidazole and polysulfone are generally prepared by a process known as solution blending. Solution blending involves dissolving both polymers in a suitable solvent to form a polybenzimidazole-containing dope. The dope can be utilized to form films or fibers by conventional techniques, or the resins blend can be precipitated out of solution utilizing a non-solvent. The precipitate can be dried to form a particulate blend which is useful for forming molded articles and the like.
When solution of polybenzimidazole and polysulfone in the range of from approximately 1 to approximately 99 weight percent of polybenzimidazole and approximately 1 to approximately 99 weight percent of polysulfone are dissolved in a suitable solvent, the polybenzimidazole will separate out of the solution after a short period of time unless it is stabilized with an organic salt, e.g., lithium chloride. While the addition of a stabilizer has lengthened the time of solution stability, it has resulted in the adverse effecting the mechanical and thermal properties of fibers spun from the dope. It has now been discovered that solutions of polybenzimidazole and polysulfone within the range of approximately 10 to approximately 35 weight percent of polysulfone and approximately 65 to approximately 90 weight percent of polybenzimidazole which are substantially free of organic salts are stable for long periods of time without phase separation. Fibers produced from the solution exhibit enhanced mechanical and thermal properties.